Actuation Cable Having Multiple Friction Characteristics

ABSTRACT

A surgical device includes a first cable portion that engages a first component such that a first friction exists between the first cable portion and the first component. The surgical device includes a second cable portion having a first end operatively coupled to a first end of the first cable portion. The second cable portion engages a second component such that a second friction exists between the second cable portion and the second component, such that the second friction is greater than the first friction.

BACKGROUND

Robotically controlled instruments are often used in minimally invasivesurgical procedures. An existing architecture for instruments in asurgical system includes an end effector or tool such as forceps, ascalpel, scissors, a wire loop, or a cauterizing instrument mounted atthe distal end of an extension, which may also be referred to herein asthe main tube of the instrument. During a surgical procedure, the endeffector and the distal end of the main tube can be inserted through asmall incision or a natural orifice of a patient to position the endeffector at a work site within the body of the patient. Tendons, whichcan be cables or similar structures, extend through the main tube of theinstrument and connect the end effector to a transmission and actuationmechanism, which may be referred to herein as a backend mechanism. Forrobotic operation of the surgical instrument, the backend mechanism atthe proximal end of the instrument is motor driven to pull on thetendons and thereby move or otherwise operate the end effector. Acomputing system may be used to provide a user interface for a surgeonor other user to control the instrument.

Certain robotically controlled surgical instruments have flexible maintubes that are able to bend as necessary to follow a natural lumen, suchas a portion of the digestive tract of a patient or for insertionthrough a curved guide tube that provides an improved approach directionto the surgical site when compared to a straight approach. Whetherinserted directly or through a guide, the main tube of a flexiblesurgical instrument will generally have multiple bends at locations thatmay vary during a surgical procedure and may vary from one procedure toanother. At these bends, the tendons running through the instrument mayrub against the inside wall of the main tube of the instrument andagainst each other, and friction generated due to these bends canincrease the forces required to move the tendons to operate the endeffector at the distal end of the main tube. Additionally, thesefrictional forces tend to be higher at zero velocity that at low (e.g.,non-zero) velocities, resulting in what is commonly called “stick-slipmotion” (and sometimes referred to as “stiction”) in response to changesin tendon load. This stick-slip motion makes smooth robotic control ofsmall movements of the end effector difficult to achieve. The largefriction also makes construction of small-diameter flexible surgicalinstruments more difficult because mechanical structures must bedesigned to be robust enough to withstand the large forces applied.

In many applications, it is desirable to provide actuation cables forcontrolling the end effector that have minimal friction to reduce thenegative impacts of friction on control of the end effector. However, incapstan drives that use friction to retain a driven cable, it isdesirable to provide an interface between the capstan surface and thecable that provides a relatively high friction to maintain the couplingbetween the cable and the capstan surface. Existing systems typicallyaddress one of the following two requirements. Using a low-frictioncable reduces the negative impacts of friction on the control of the endeffector, but reduces the force that can be applied by the capstan dueto slippage of the low-friction cable. Alternatively, using ahigh-friction cable impedes the control of the end effector, butincreases the force that can be applied by the capstan due to reducedslippage of the high-friction cable.

Accordingly, instruments are desired that provide a cable offeringsufficiently low friction to enable control of an end effector as wellas significant friction between the cable and a capstan surface toproperly drive the cable.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a surgical instrument in accordance with an embodiment ofthe invention that includes a cable having two different frictioncharacteristics.

FIG. 2 depicts a cable in accordance with an embodiment of the inventionpartially covered by a lower friction material.

FIG. 3 depicts a cable in accordance with an embodiment of the inventioncovered by two different materials.

FIG. 4 depicts a surgical instrument in accordance with an embodiment ofthe invention that includes a cable having two portions coupled to oneanother.

FIG. 5 depicts a coupling mechanism in accordance with an embodiment ofthe invention that releasably couples two cable portions.

FIG. 6 depicts a coupling mechanism in accordance with anotherembodiment of the invention that releasably couples multiple tendons ina flexible device.

FIGS. 7A and 7B depict a portion of a surgical instrument in accordancewith an embodiment of the invention using springs and capstan frictionto maintain tension on tendons in the instrument.

FIG. 8 depicts a portion of a flexible surgical instrument in accordancewith an embodiment of the invention using a coupling mechanism thatreleasably couples two instrument portions having multiple tendons.

FIG. 9 depicts a portion of a flexible cable using a sheath and a tendonpassing through the sheath.

Throughout the description, similar reference numbers may be used toidentify similar elements.

DETAILED DESCRIPTION

The systems and methods described herein relate to surgical instrumentshaving a cable, an end effector and a backend mechanism to operate theend effector via the cable. In a described embodiment, the cable isformed using two different materials having different frictioncharacteristics. A first cable portion is connected to the end effectorand has low friction properties that provide desirable control of theend effector. A second cable portion is connected to the backendmechanism and has high friction properties that permit the applicationof significant force to the cable by the backend mechanism. Inparticular, the second cable provides high friction characteristics whenengaged with the surface of a drive mechanism, such as a capstan. Manyaspects and embodiments are described with reference to a “cable” or a“cable portion”. It should be understood, however, that a cable isrepresentative of a “tension element” or “tendon” that is capable oftransmitting a pulling force, and such components may be various cables,wires, rods, fibers, threads, and the like. Persons having ordinaryskill in the art will understand that such tension elements/tendons insome instances may also be capable of transmitting a pushing force.

FIG. 1 depicts a surgical instrument 100 in accordance with anembodiment of the invention that includes a cable having two differentfriction characteristics. Surgical instrument 100 includes a lengthwisefirst cable portion 102 having low friction properties and a secondlengthwise cable portion 104 having high friction properties whenengaging a surface of a drive mechanism. In a particular embodiment,cable portion 102 is manufactured using a synthetic polymer, such ashigh density polyethylene (HDPE). In a specific implementation, cableportion 102 is manufactured using Dyneema®. In the described embodiment,cable portion 104 is manufactured using a different synthetic materialhaving a higher coefficient of friction than the material used for cableportion 102. Example materials for cable portion 104 include an aramidpolymer (such as Kevlar®) or PBO(poly(p-phenylene-2,6-benzobisoxazole)).

A coupling element 106 operatively couples cable portion 102 and cableportion 104. Coupling element 106 is any type of coupling mechanism thatsecurely joins the two lengthwise cable portions 102 and 104 end to end.Examples of coupling element 106 include a thimble, an eye, a crimpelement, and the like. In alternate embodiments, cable portion 102 isoperatively coupled to cable portion 104 by splicing the two cableportions together or braiding/weaving the two cable portions to oneanother. In other embodiments, cable portion 102 is operatively coupledto cable portion 104 using a knot or similar method of securing the twocable portions to each other.

In one aspect, the distal end of cable portion 102 is connected to adistal component of the instrument, such as distal link or a surgicalend effector 108, which can perform various procedures, such as cutting,removal or destruction of tissue, insertion of medical devices,cauterization, vessel sealing, suturing, and the like. Many aspects andembodiments are described in terms of an end effector, and it should beunderstood that such an end effector is representative of varioussurgical instrument distal components, such as links in a kinematicchain, wrist mechanisms, end effectors, and the like. In the embodimentof FIG. 1, the low friction properties of cable portion 102 providedesirable control of the end effector by generally avoiding stick-slipmotion. In a particular implementation, cable portion 102 includes alubricated tendon that passes through a sheath, as discussed below withrespect to FIG. 9.

In a highly articulated flexible instrument, the total aggregatefriction of all of the bends, typically referred to as capstan friction,also greatly degrades the performance of the instrument and is typicallyapproximated as an exponential function of the product of thecoefficient of friction and the total angle of wrap. Such capstanfriction may result from the cable being routed through one or moreguide channels, such as a fairlead, in jointed distal links or past oneor more pulleys or so that the cable follows a tortuous path to itstermination point. In a particular embodiment, the distal cable portion102 has both a static coefficient of friction less than or equal to itsdynamic coefficient of friction and a sufficiently low dynamiccoefficient of friction to allow effective force transmission. As shownin FIG. 1, two components 116 and 118 interact with cable portion 102,generating friction as a result of that interaction. In variousembodiments, components 116 and 118 are drive mechanisms, capstans,pulleys, guide channels or other mechanisms that operatively interactwith cable portion 102.

The proximal end of cable portion 104 is connected to a backendmechanism 110, which includes a capstan 112 and a tension spring 114.Cable portion 104 wraps around capstan 112 and is held in place by thefriction between the surface of cable portion 104 and the surface ofcapstan 112. The tension spring 114 maintains tension on cable portion104. In the described embodiment, capstan 112 is a powered capstan, asdiscussed below with respect to FIGS. 7A and 7B. In alternateembodiments, capstan 112 is replaced with any type of drive mechanism,friction drive or other driving mechanism that uses friction.

A particular implementation of surgical instrument 100 includes a singlebackend mechanism 110 having multiple capstans 112 connected to multiplecable portions 104, which are coupled to multiple cable portions 102 andcorresponding multiple end effectors 108. In this implementation,multiple cable portions 102 are individually sheathed, then bundled androuted through a single main tube. Aspects of this capstan and cablecombination are disclosed in U.S. patent application Ser. No. 12/286,644(filed Sep. 30, 2008; entitled “Passive Preload and Capstan Drive forSurgical Instruments”; published as U.S. Patent Application Pub. No. US2010/0082041 A1), which is incorporated herein by reference.

As discussed below, certain embodiments of cable portion 102 use atendon routed through a sheath such that a lubricant is located betweenthe tendon and the sheath. The lubricant maintains a low friction in thecable portion 102, which is desirable for control of the end effector.However, such lubrication of cable portion 104 is not desirable due to apotential of reduced friction between the cable and capstan 112 surface.In certain embodiments, coupling element 106 between cable portions 102and 104 helps maintain the proper operation of surgical instrument 100by isolating the lubricant in cable portion 102, where it is desirableto operation of the surgical instrument.

FIG. 2 depicts a cable 202 partially covered by a lower frictionmaterial 204 (also referred to as a “covering material”) in accordancewith an embodiment of the invention. In this embodiment, cable 202 hashigh friction characteristics (as desired when interacting with acapstan) and material 204 has a lower friction characteristic (asdesired when operating a surgical instrument). In a particularembodiment, material 204 is a braided material that covers cable 202 inthe area that does not make contact with a capstan surface. Instead, thehigher-friction cable 202 contacts the capstan surface. In thisembodiment, cable 202 is manufactured using Kevlar® or PBO, and material204 is manufactured using HDPE, such as Dyneema®. In an alternateembodiment, cable 202 is manufactured using Dyneema® and material 204 ismanufactured using Kevlar® or PBO. In this alternate embodiment, theportion of the cable covered with material 204 contacts the capstansurface. In a particular implementation, material 204 is under tension,which causes material 204 to apply pressure on cable 202. This pressurereduces the possibility of slippage between material 204 and cable 202.

FIG. 3 depicts a cable 302 covered by a first material 304 and a secondmaterial 306 in accordance with an embodiment of the invention, wherematerial 304 has high friction characteristics and material 306 has lowfriction characteristics. Materials 304 and 306 are also referred to as“covering materials”. In this embodiment, material 304 contacts acapstan surface and material 306 operates or controls an end effector orother distal instrument component. Material 304 and material 306 meet atan interface point 308, which operatively couples the two materials.Materials 304 and 306 can be coupled by braiding/weaving the twomaterials, splicing the two materials, using a coupling element tooperatively couple the materials, and so forth. In a particularembodiment, material 304 is Kevlar® or PBO, material 306 is HDPE, suchas Dyneema®, and cable 302 is manufactured from a material having highstrength and high elastic modulus, such as PBO.

FIG. 4 depicts a surgical instrument 400 in accordance with anembodiment of the invention that includes a cable having two portionsreleasably coupled to one another. The surgical instrument shown in FIG.4 is similar to the embodiment of FIG. 1, but it includes a releasablecoupling element. Surgical instrument 400 includes a first cable portion402 (also referred to as a first tendon) having low friction propertiesand a second cable portion 404 (also referred to as a second tendon)having high friction properties when engaging a capstan surface. In aparticular embodiment, cable portion 402 is manufactured using asynthetic polymer, such as Dyneema®. In that embodiment, cable portion404 is manufactured using a different synthetic material having a highercoefficient of friction. Example materials for cable portion 404 includean aramid polymer (such as Kevlar®) or PBO.

Cable portion 402 has a coupling element 408 connected at a proximal endthereof. Cable portion 404 has a coupling element 406 connected at adistal end thereof. Coupling elements 406 and 408 operatively couplecable portions 402 and 404, and they are configured to releasably engagewith one another. As discussed herein, coupling elements 406 and 408apply movement and forces on cable portion 404 to cable portion 402, andvice versa. Coupling elements 406 and 408 are securely mounted orotherwise connected to cable portions 404 and 402, respectively.

The distal end of cable portion 402 is connected to an end effector 410,which can perform various surgical procedures, such as cutting, removalor destruction of tissue, insertion of medical devices, cauterization,vessel sealing, suturing, and the like. In the embodiment of FIG. 4, thelow friction properties of cable portion 402 provide desirable controlof the end effector by generally avoiding stick-slip motion withreference to other instrument components as cable portion 402 extendsthrough the instrument. In a particular implementation, cable portion402 includes a lubricated tendon passing through a sheath, as discussedbelow with respect to FIG. 9.

The distal end of cable portion 404 is connected to a backend mechanism412, which includes a capstan 414 and a tension spring 416. Alternateembodiments may use any type of tension mechanism in place of tensionspring 416, such as a torsion spring driving a second take-up capstan towhich the end of cable portion 404 is terminated. In this alternateembodiment, the take-up capstan may be shaped to linearize the torsionspring. Cable portion 404 wraps around capstan 414 and is held in placeby the friction between cable portion 404 and the surface of capstan414. The tension spring 416 maintains tension on cable portion 404. Inthe described embodiment, capstan 414 is a powered capstan, as discussedbelow with respect to FIGS. 7A and 7B. A particular implementation ofsurgical instrument 400 includes a single backend mechanism 412 havingmultiple capstans 414 connected to multiple cable portions 404, whichare then coupled (via multiple coupling elements 406 and 408) tomultiple cable portions 402 and corresponding multiple end effectors410. In this implementation, multiple cable portions 402 are bundled androuted through a single main tube.

The releasable coupling between cable portions 402 and 404 permits thesimple and easy replacement of cable portion 402. In particularimplementations, cable portion 402 is replaced after a particular numberof surgical procedures or a particular time period. This replacementallows cable portion 402, which may be subject to relatively higher wearthan cable portion 404, to be replaced, thereby reducing overallinstrument cost for the effective life of the instrument. Offering aquick-release coupling between cable portions 402 and 404 facilitatesthis replacement. Additionally, the releasable coupling between cableportions 402 and 404 isolates any lubricant used in cable portion 402from cable portion 404, which maintains the desired higher frictionbetween cable portion 404 and capstan 414. As discussed below, certainembodiments of cable portion 402 use a tendon routed through a sheathsuch that a lubricant is located between the tendon and the sheath. Thelubricant maintains a low friction in the cable portion 402, which isdesirable for control of the end effector. However, such lubrication ofcable portion 404 is not desirable due to a potential of reducedfriction between the cable and the capstan surface. Thus, the couplingbetween cable portions 402 and 404 helps maintain the proper operationof surgical instrument 400 by isolating the lubricant in cable portion402, where it is desirable for operation of the surgical instrument.

FIG. 5 depicts a coupling mechanism 500 in accordance with an embodimentof the invention that releasably couples two cable portions. Couplingmechanism 500 is an alternative to the releasable coupling elements 406and 408 discussed above with respect to FIG. 4. A first cable portion502 has a ball 506 attached to the proximate end thereof. A second cableportion 504 has an arm 508 attached to the proximate end thereof. Arm508 includes a recessed portion 510 that corresponds in shape to theouter surface of ball 506. Arm 508 slidably engages a cam 512 thatallows arm 508 to rotate about cam 512 when engaging ball 506. Forexample, as ball 506 slides between arm 508 and surface 514, arm 508rotates about cam 512 to allow ball 506 to continue sliding towardrecessed portion 510. As ball 506 aligns with recessed portion 510, arm508 rotates back toward a position that is substantially parallel withsurface 514, thereby engaging ball 506 in recessed portion 510. Onceball 506 and arm 508 are engaged, movement of second cable portion 504causes a corresponding movement in first cable portion 502 via arm 508and ball 506. First cable portion 502 can be released from second cableportion 504 by rotating arm 508 to disengage ball 506, thereby allowingball 506 to be slidably released from arm 508.

FIG. 6 depicts a coupling mechanism 600 in accordance with anotherembodiment of the invention that releasably couples multiple tendons ina flexible device. Coupling mechanism 600 is utilized to couple a firstcable portion 602 to a second cable portion 604. As discussed herein,cable portion 604 is coupled to a capstan or other actuating mechanismthat moves cable portion 604. Multiple tendons extend through cableportions 602 and 604. Coupling mechanism 600 includes a first couplingelement 606 connected to cable portion 602, and a second couplingelement 608 connected to cable portion 604. In the example of FIG. 6,the coupling mechanism 600 supports six tendons bundled together in asingle tube or similar structure. The multiple tendons are bundled intoa single main tube represented by cable portion 602. The distal ends ofthe multiple tendons are each connected to an end effector.

FIG. 6 illustrates a particular coupling mechanism. Alternateembodiments may utilize any type of coupling mechanism that couplesmultiple tendons in a flexible device.

The coupling mechanisms shown in FIGS. 4, 5, and 6 represent threeexamples of mechanisms to releasably couple a first cable portion to asecond cable portion. Alternate embodiments may include any type ofcoupling mechanism that allows the interaction between the first andsecond cable portions as discussed herein. Further, alternateembodiments may include any type of material and any configuration orarrangement of components. Additional aspects of cable couplingmechanisms are disclosed in U.S. patent application Ser. No. 12/425,272(filed Apr. 16, 2009; entitled “Tendon-Driven Endoscope and Method ofUse”; published as U.S. Patent Application Pub. No. US 2010/0094088 A1)and U.S. Patent Application No. 10/988,212 (filed Nov. 12, 2004;entitled “Connector Device for a Controllable Instrument”; published asU.S. Patent Application Pub. No. 2006/0052664 A1), both of which areincorporated herein by reference.

FIGS. 7A and 7B depict a portion of a surgical instrument 700 usingsprings and capstan friction to maintain tension on tendons in theinstrument in accordance with an embodiment of the invention. FIG. 7Aillustrates a top view of backend mechanism 710 connected to four cableportions 702, 704, 706, and 708 (also referred to as “tendons”). Backendmechanism 710 includes four capstans 712, 714, 716, and 718, each ofwhich is engaged with one of the four cable portions 702-708. Cableportions 702-708 apply forces through a coupling and another cableportion to manipulate an end effector (not shown), as described above.Cable portions 702-708 are engaged with capstans 712-718 by wrappingeach of the cable portions around a particular capstan, such that theresulting friction between the cable portions and the capstan surfacesmaintains the engagement to provide an effective actuating pull force tothe cable sufficient to operate the associated distal end component.

Backend mechanism 710 also includes tension springs 720, 722, 724, and726 that are attached to the ends of cable portions 702-708,respectively, to maintain tension on cable portions 702-708. Springs720-726 are biased (e.g., stretched) to apply a non-zero force to cableportions 702-708. In alternate embodiments, the springs used in backendmechanism 710 are rotary coil springs, leaf springs, or compliantmembers, such as bending beams, cantilever beams, or elastic bands.Other embodiments utilize a torsion spring driving a second take-upcapstan. In these embodiments, the take-up capstan may be shaped tolinearize the torsion spring.

FIG. 7B illustrates a side view of backend mechanism 710 and two drivemotors 730 and 732 that engage with the capstans in the backendmechanism. Drive motors 730 and 732 are under the active control ofhuman input and software executed in a teleoperated surgical system.FIG. 7B illustrates drive motors 730 and 732 that are coupled tocapstans 716 and 718, respectively. When activated, drive motors 730 and732 apply a rotational force to capstans 716 and 718, which in turnapply that force to cable portions 708 and 706 to retract the cable orrelax the cable depending on the direction of rotation.

FIG. 8 depicts a portion of a flexible surgical instrument 800 inaccordance with an embodiment of the invention using a couplingmechanism that releasably couples two instrument portions havingmultiple tendons extending therethrough. In alternate embodiments,instrument 800 includes a non-releasable coupling element, such ascoupling element 106 shown in FIG. 1. Instrument 800 is capable ofbending into various shapes as may result during a medical procedurewhen the instrument follows a curved path inside a patient to a sitewhere a surgical procedure (or other medical or diagnostic procedure)may be performed. The path to the site may extend through an incision orthrough a natural orifice of a patient and along a natural lumen, suchas a portion of the digestive tract of a patient. Portions of instrument800 may further pass through an incision in the wall of the naturallumen to access the surgical site or further portions of the path thatthe distal end of the instrument must follow. Instrument 800 willgenerally have a different shape during different procedures and mayneed to follow a convoluted path including one or more bends.

Instrument 800 includes a first instrument portion 802 and a secondinstrument portion 804 coupled to one another via coupling elements 806and 808. Coupling element 806 is connected to the proximate end of firstinstrument portion 802. One or more end effectors (not shown) areconnected to the distal end of first instrument portion 802. Couplingelement 808 is connected to the proximate end of second instrumentportion 804. A backend mechanism 810 is connected to the distal end ofsecond instrument portion 804. Coupling elements 806 and 808 may be ofthe type described above in FIG. 6 or any other appropriate couplingmechanism.

First instrument portion 802 includes three tendons 812, 814, and 816positioned therein. Tendons 812, 814, and 816 are flexible tendonscapable of flexing and bending as first instrument portion 802 is movedand flexed as a result of various manipulations and procedures. Tendons812, 814, and 816 interact with a corresponding three tendons 818, 820,and 822 positioned in the relatively more proximal second instrumentportion 804. In the embodiment of FIG. 8, tendons 812, 814, and 816 aremanufactured using a first type of material, and tendons 818, 820, and822 are manufactured using a second type of material. For example, asdiscussed above, tendons 812, 814, and 816 are manufactured usingDyneema®, and tendons 818, 820, and 822 are manufactured using Kevlar®or PBO. In this example, tendons 812, 814, and 816 are each surroundedby a stainless steel sheath of the type discussed below with respect toFIG. 9.

In specific implementations, the Dyneema® cable has a diameter ofapproximately 0.5 millimeter and has a desired coefficient of frictionless than 0.10 between the cable and the sheath. Particular embodimentshave a desired coefficient of friction in the range of 0.03-0.05 betweenthe cable and the sheath. The Kevlar® or PBO material has a diameter ofapproximately 0.5 millimeter and has a desired coefficient of frictionin the range of 0.30-0.50 between the cable and the surface of thecapstan.

Although FIG. 8 illustrates three tendons positioned within firstinstrument portion 802 and second instrument portion 804, alternateembodiments may include any number of tendons positioned within thefirst and second instrument portions. In a specific implementation, oneor more of the tendons are used to articulate intermediate sections ofthe instrument. The number of tendons contained in a particular flexiblesurgical instrument depends, in part, on the control and/or motionrequirements of the end effector connected to the flexible surgicalinstrument. In particular embodiments, the end effector can perform avariety of functions and includes a wrist mechanism and/or a graspingmechanism. It should also be understood that the depicted first andsecond instrument portions are illustrative, and the combined first andsecond instrument portions do not necessarily have to encompass theentire instrument length. One or more additional lengthwise instrumentcomponents may be placed between and/or beyond ends of the depicted anddescribed first and second instrument portions.

FIG. 9 depicts a portion of a flexible cable 900 having a sheath 902 anda tendon 904 passing through the sheath. The portion shown in FIG. 9 ispart of the first, more distal, instrument portion discussed herein;i.e., the instrument portion with an end effector at the distal end anda coupling element at the proximate end. Cable 900 may contain air or alubricant to reduce the friction associated with movement of tendon 902through cable 900. As discussed herein, a particular embodiment of cable900 includes tendon 904 manufactured using HDPE, such as Dyneema®, andsheath 902 manufactured using stainless steel. Specific embodiments usea lubricant that is a mixture of water, a fatty acid or refined mineraloil, and a suitable surfactant to reduce friction in cable 900. Sheath902 may be porous to permit movement of the lubricant between theinterior and exterior of the sheath. In certain embodiments, seals suchas O-rings or bellows-type seals can keep the lubricant within a sealedportion of sheath 902. Additional tendon lubrication information isdisclosed in U.S. patent applications Ser. Nos. 12/346,402 (filed Dec.30, 2008; entitled “Surgical Instrument with Sheathed Tendons”;published as U.S. Patent Application Pub. No. US 2010/0168510 A1) and12/346,461 (filed Dec. 30, 2008; entitled “Lubricating Tendons in aTendon-Actuated Surgical Instrument”; published as U.S. PatentApplication Pub. No. US 2010/0168721 A1), both of which are incorporatedherein by reference.

The described combination of sheath 902 material and tendon 904material, as well as any lubricant, provides low friction and generallyavoids stick-slip motion. In other embodiments, different metals or highstrength polymers can be substituted for stainless steel in sheath 902.Similarly, in other embodiments, different materials can be substitutedfor Dyneema® in tendon 904.

Embodiments of the systems and methods described herein relate tosurgical instruments having a cable, an end effector, and a backendmechanism to operate the end effector via the cable. Certain embodimentsare used in conjunction with one or more conventional surgical systemsand methods. For example, one embodiment is used as an improvement ofexisting surgical systems.

Although the components and modules illustrated herein are shown anddescribed in a particular arrangement, the arrangement of components andmodules may be altered to implement surgical instruments in a differentmanner, or to manipulate surgical systems in a different manner. Inother embodiments, one or more additional components or modules may beadded to the described systems, and one or more components or modulesmay be removed from the described systems. Alternate embodiments maycombine two or more of the described components or modules into a singlecomponent or module.

Although specific embodiments of the invention have been described andillustrated, the invention is not to be limited to the specific forms orarrangements of parts so described and illustrated. The scope of theinvention is to be defined by the claims appended hereto and theirequivalents.

1. A surgical device comprising: a first cable portion engaging a firstcomponent such that a first friction exists between the first cableportion and the first component; and a second cable portion having afirst end operatively coupled to a first end of the first cable portion,the second cable portion engaging a second component such that a secondfriction exists between the second cable portion and the secondcomponent, wherein the second friction is greater than the firstfriction.
 2. The surgical device of claim 1, wherein the first cableportion includes a tendon extending through a sheath.
 3. The surgicaldevice of claim 2, wherein the tendon comprises high densitypolyethylene.
 4. The surgical device of claim 2, wherein the sheathcomprises stainless steel.
 5. The surgical device of claim 1, whereinthe second cable portion comprises an aramid polymer.
 6. The surgicaldevice of claim 1, wherein the second cable portion comprisespoly(p-phenylene-2,6-benzobisoxazole).
 7. The surgical device of claim1, wherein the second cable portion is operatively coupled to the firstcable portion using a releasable coupling element.
 8. The surgicaldevice of claim 1, wherein the first cable portion is configured to beconnected to an end effector.
 9. The surgical device of claim 1, whereinthe second component is a powered capstan.
 10. The surgical device ofclaim 1, wherein the first friction has an associated coefficient offriction less than 0.10.
 11. The surgical device of claim 1, wherein thesecond friction has an associated coefficient of friction greater than0.3. 12-21. (canceled)
 22. The surgical device of claim 1, wherein thefirst cable portion is operatively coupled to the second cable portionalong a common lengthwise axis defined through the first and secondcable portions.
 23. The surgical device of claim 1, wherein the firstcable portion includes a cable and a covering material extending alongthe cable.
 24. The surgical device of claim 23, wherein the coveringmaterial is a braided covering material.
 25. The surgical device ofclaim 23, wherein the covering material is under tension and appliespressure to the cable.